Many rubber articles, principally automobile tires, but also including hoses, conveyor belts, transmission belts, and the like are usually reinforced with fibers in cord form. In all such instances the fiber must be firmly bonded to the rubber. This is so whether the fiber is natural, synthetic or metallic, or whether the rubber is natural or synthetic.
The conventional practice has been to prepare the fiber by pretreatment with a rubber latex and a phenol-formaldehyde condensation product, wherein the phenol is almost always resorcinol. By a mechanism not completely understood, the resin reacts with the fiber and the rubber, effecting a firm reinforcing bond.
An alternative method entails compounding a vulcanizable rubber stock composition with the components of the phenol formaldehyde condensation product. The components of the condensation product consist of a formaldehyde, or methylene, acceptor and a formaldehyde, or methylene, donor. The most commonly employed methylene acceptor is resorcinol, while the more commonly employed methylene donors are the N-(substituted oxymethyl)melamine. The effect achieved is resin formation in situ during vulcanization of the rubber, creating a bond between the fiber and rubber irrespective of whether the fiber has been pretreated. This method is particularly useful with steel cord, where pretreatment has been largely ineffective.
However, the use of resorcinol has many inherent disadvantages. One of the most frequently encountered is occasioned by the fact that resorcinol is not readily dispersed in rubber. In addition, resorcinol is toxic. The most commonly encountered side effect of its use is the dermatological reaction commonly referred to in the trade as "red hand". These difficulties are further compounded by the fact that there is a worldwide shortage of resorcinol.
In addition, an increased need in the industry for fiber reinforcing of rubber to survive high dynamic stress, such as flexing, has brought about a continuing search for other and better methods for achieving high adhesive strength.
The present invention represents the culmination of a long series of investigations, conducted largely by the inventor, directed to finding a vulcanizable rubber stock composition comprising the components of the phenol-formaldehyde condensation product which avoids the use of resorcinol as the methylene acceptor.
Accordingly, it is a primary object of the present invention to provide a vulcanizable rubber stock composition comprising the components of the phenol-formaldehyde condensation product without resorting to the use of resorcinol as a methylene acceptor.
Another object of the invention is to provide a vulcanizable rubber stock composition which provides equal or better results for the bonding of fibers to rubber.
Generally then, it is an object of the invention to provide a new and improved vulcanizable rubber stock which avoids the inherent disadvantages of employing resorcinol as the methylene acceptor, as well as providing equal or better bonding results.
Additional objects and advantages of the invention will be set forth in part in the broad description which follows, and in part will be obvious from the description, or may be realized by practice of the invention, the objects and advantages being realized and attained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.